The Brief Pain Inventory (BPI) was developed for rapid assessment of the severity and impact of pain in cancer patients. It is presently widely used in both research and clinical settings world-wide to assess chronic, non-malignant pain caused by a variety of clinical conditions, as well. The BPI allows patients to rate the severity of their pain and the degree to which their pain interferes with common dimensions of feeling and function. It has been used in hundreds of studies. The scale is available in two formats: the BPI short form, which is used for clinical trials and is the version used for the foreign-language translations; and the BPI long form, which contains additional descriptive items that may be clinically useful (for example, items that expand the possible descriptors of pain, such as burning, tingling, etc.). For brevity’s sake and for the patient’s ease of use, however, the developers recommend the short form of the BPI. In response to the FDA draft guidance for the pharmaceutical industry on the use of patient-reported outcomes (PRO) measures in medical product development to support labeling claims, we have prepared a BPI User's Guide to provide documentation of the BPI's development and psychometric properties. The information offered therein addresses the recommendations in the FDA draft guidance and establishes the BPI's adequacy as a measure to support medical product claims.